1. Field of the Invention
The present invention relates to the field of positively acting differentials for motor vehicles.
2. Prior Art
Land vehicles, such as automobiles, trucks, buses and the like, typically utilize what has become known as an "open differential" for the final drive system. In such a differential, bevel gears are coupled to the inner ends of left and right collinear axles. These bevel gears engage accompanying bevel gears mounted on a pinion pin in a differential case. The differential case, in turn, has a ring gear thereon, with the ring gear and differential case being driven in rotation about the axis of the axles by a pinion gear on the drive shaft. The angular velocity of the ring gear and differential case determines the average angular velocity of the two axles. However, the bevel gearset within the differential case between the two axles allows one axle to turn faster and the other axle to turn slower than the ring gear and differential case at any particular time. This, of course, is highly desirable in normal driving, as it allows the axle coupled to the outer wheel to rotate faster than the axle coupled to the inner wheel when going around a curve or turning a sharp corner. It also causes the drive system to deliver the same drive torque to each of the two axles to avoid a tendency for the vehicle to pull to one side or the other when power is applied or removed. The amount of torque that can be transmitted through an open differential is limited to that able to be carried by the wheel with the least amount of traction.
There are certain situations, however, where the aforementioned characteristics of an open differential become undesirable. In particular, when one wheel loses traction, the torque which will be delivered to the wheel with traction will be no higher than the torque delivered to the wheel without traction. For instance, with one drive wheel on ice and the other drive wheel on dry concrete, the torque delivered to the drive wheel on dry concrete will be no higher than can be carried by the wheel on ice. A locking differential, however, will effectively couple the two axles together so that they turn in unison, forcing rotation of the drive wheel with greater traction along with the rotation of the wheel with lesser traction. The locking differential, as opposed to the open differential, can transmit as much torque as can be carried by the wheel with the most traction. High performance vehicles, off-road vehicles and the like may similarly take advantage of the characteristics of locking differentials to improve their traction performance.
The positive acting differential of the present invention is a significant improvement in a prior art locking differential manufactured by Vehicular Technologies, Inc., assignee of the present invention, and sold under the trademark "Performance." That prior art device is shown in the exploded view of FIG. 1.
Referring to FIG. 1, the splined inner end of axle 20 engages mating splines in a coupler 22, with a similar coupler 22 at the opposite side of the assembly similarly mating at the inner end of the other axle, not shown. The axle bears against a pinion pin 34 that provides the inward limit for the axial position of the axle. In some embodiments the ends of the axles 20 bear against the inner face of the spacer and the spacer in turn bears against the pinion pin to establish the desired inward limit. A locking differential reuses some components of the open differential supplied with the vehicle. In particular, the pinion pin 34 that carries the bevel gears of the open differential may be reused although the bevel gears are not used. For this reason, pin 34 is referred to as a pinion pin even though it does not carry gears in a positive acting differential of the type shown. In the version shown, the axles are retained in position by C clips 24 that establish the outward limit for the axial position of the axle.
The couplers 22 have a plurality of teeth 26 on the face thereof which may mate with corresponding teeth on the faces 28 of drivers 30, depending upon the axial position of the drivers. The drivers 30, in turn, have saddle-shaped depressions 32 on the opposite faces thereof for loosely surrounding the pinion pin 34 driven by the differential case 54 (see FIG. 2 for the position of the pinion pin in the overall differential assembly). The drivers 30 each have springs 36 in angled blind holes in the driver, the springs acting on pin 34 to both elastically encourage the drivers to a position having the pin 34 aligned with the center of the saddle-shaped depressions, and to elastically encourage the drivers axially outward away from the pin 34 into engagement with the couplers. Pins 40 on the drivers 30 fit within slots 42 on the opposing face of the opposite driver and function to control the angular displacement of the drivers to each other.
The drivers 30 must be in close axial alignment with the couplers 22 and be free to move axially to allow engagement and disengagement from the adjacent coupler to provide the locking differential action. The outer diameters of the splined ends of the axles 20 typically do not provide a suitable locating surface for the drivers. Spacers 38 establish and retain the drivers 30 in axial alignment with the couplers 22 and provide sliding surfaces for the drivers. As may be seen in FIG. 4, each spacer is located relative to a coupler by a radial shoulder in the face of the coupler. While FIG. 4 shows the spacer aligned by a shoulder against an inside diameter of the spacer, it will be appreciated that the spacer can also be aligned by a shoulder against an outside diameter. The spacers are closely fitted between the pinion pin 34 and the adjacent coupler 22 to maintain the axial position of the spacer. However, there is sufficient clearance to allow the spacers to rotate relative to the couplers.
In the final assembly, the springs 36 encourage the toothed face of the drivers 30 into engagement with the toothed face of couplers 22, and there is sufficient clearance between the saddle-shaped depressions 32 and pin 34 in the final assembly for either driver to move toward the pin 34 sufficiently to allow the teeth of a driver 30 to ride over the teeth of the associated coupler 22.
The operation of the prior art device may be explained as follows. With the teeth of the corresponding driver and coupler pairs engaged, the differential housing may rotate, carrying pin 34 from contact with one side of the saddle to the other, a displacement of (depending on the size of the design) 4 to 7 degrees. This free travel, or backlash, is essential for correct positioning of the differential components during the transition from driving to coasting and vice versa. The drivers are retained with respect to each other by pins 40 and mating slots 42 for a total rotation, one relative to the other, approximately one half of the backlash described previously. When the pin 34 engages the saddle-shaped depressions 32 on either driver, the force of the contact, by design of the saddles, will be angled outward from the plane of the respective driver and will overcome the component of the reaction force acting opposite created by the inclined edges on the mating teeth on the drivers 30 and couplers 22. For example, saddle angles ranging from 30 to 40 degrees are typically used and create outward axial forces that exceed the inward axial forces created by typical 20 to 25 degree inclines of the coupler and driver mating teeth that would otherwise work to separate the driver from the coupler. Using the foregoing parameters, consider first the vehicle at rest. Assume the two drivers 30 each engage with the respective coupler 22, and for specificity in the starting condition, that the pin 34 is centered in the saddle-shaped depressions 32 in the drivers 30. With the vehicle in gear and engine driving, the pin 34 begins to rotate about the axis of the axle, through the backlash present and compressing against springs 36 to contact the edges of the saddle-shaped depressions 32 in the drivers, and then on further rotation, to force the drivers and couplers, and thus the axles, into rotation. Because the contact angle between the pin 34 and the saddle-shaped depressions 32 exceeds the angle of the edge of the teeth on the couplers and drivers, the force between the pin and the drivers forcing the same into contact against the couplers 22 will exceed the force between the inclined edges of the teeth on the drivers 30 and couplers 22 otherwise tending to force the drivers back toward pin 34, so that the drivers and couplers will remain in positive engagement, regardless of the torque applied to the differential.
If the vehicle now proceeds to drive around a curve, the wheel on the outside of the curve, and thus the coupler 22 associated with that wheel, will tend to rotate faster than the coupler associated with the inside wheel. Assuming power is still being applied, this causes the driver associated with the outside wheel to begin "gaining" with respect to pinion pin 34, the driver rotating forward to a position wherein the saddle-shaped depressions 32 thereon are no longer in contact with pin 34. At this point, pins 40 and mating slots 42 prevent the further relative rotation of the two drivers but allow coaxial translation. Further gaining of the outside wheel continues to rotate the outside coupler at a speed higher than the other differential components. Now, however, the teeth on the driver associated with the outside wheel are free to climb the inclined planes of the teeth on the driver and coupler, with the driver moving toward the pin 34 against the resistance of the associated springs 36 to allow the teeth of the respective driver to slide over the teeth of the respective coupler, repeatedly as required so long as the difference in wheel rotation speeds exist.
If, when in a curve, the vehicle engine is throttled back to coast and the engine is used as a braking or vehicle slowing device, the same basic interaction of parts described above will occur substantially in reverse, now however with the driver and coupler associated with the outer wheel of the curve being engaged, and the driver associated with the inner wheel of the curve climbing over the teeth on the associated coupler as required to allow the inner wheel on the curve to turn slower than the outer wheel. Similarly, in backing around a curve such as backing out of a parking place, the inner wheel will be the drive wheel, as in powering forward, whereas use of the engine to retard the motion of the vehicle when backing will engage the wheel on the outer side of the turn. However in any event, when power is applied while turning to the point that traction is lost by the drive (inside) wheel, pin 34 will catch up to and forcibly engage the appropriate side of the saddle-shaped depression 32 on the outside wheel driver 30, forcing both drivers into engagement with their associated couplers to force rotation of both axles in unison.
The foregoing locking differential and another manufactured by Vehicular Technologies called "Lock Right" perform well, allowing the drive wheels to rotate independently under normal conditions, but causing the wheels to rotate in unison when either wheel loses traction and begins to slip. The Lock Right design differs from the aforementioned design in that it has no springs located in the saddle to dampen the backlash, rather springs are located between the drivers and thus work directly to force the driver teeth into mesh with those on the adjacent coupler. However, these differential designs contain a few particular operating characteristics that may require the vehicle operator to become accustomed to.
In particular, when one wheel begins turning faster than the other, such as when turning into a parking space, one driver will be climbing the teeth on the associated coupler and sliding thereover. When the teeth of the driver again align with the spaces between teeth on the coupler, the driver will fall into engagement with the coupler and shortly thereafter climb the sides of the teeth and again disengage. This makes an audible noise, resulting in a "click, click, click" type sound heard from outside the vehicle. In louder vehicles, such as a high performance vehicle, particularly for one technically versed to understand the source of the sound, the sound is of little consequence. However, to the driver of a more typical, quieter car, the sound can be a distraction, and could be misinterpreted as a mechanical fault or impending mechanical failure. Secondly, a phenomenon called "cycling" can be induced in manual transmission equipped vehicles. Automatic transmissions do not exhibit the condition because the torque converter always maintains a measure of bias load between the engine and drive axle. With manual transmissions, this event occurs when turning at slow speeds with the clutch pedal depressed, such as when turning into a parking space, temporarily decoupling the transmission from the engine and therefore removing any bias load present on the engaged driver and coupler. When the disengaged driver and coupler teeth pass by each other, they briefly reengage, enabling a load to be placed on the differential and axle components. The components between the differential and the wheel then act like an undamped mechanical spring and release the energy by temporarily accelerating the differential, drive shaft and transmission components. The inertia of these components carries the differential pin against the driver saddle, causing the opposite side driver and coupler to lock and continue to process. The continual wind-up and release will build and eventually become sufficient to "rock" the vehicle driveline and require the transmission be put in neutral or the vehicle stopped in order to cease the cycling. Needless to say, this is highly undesirable and would only be acceptable to the very most forgiving of owners. However all drivers whose vehicles represent a standard "as-delivered" condition would appreciate the increased traction a locking differential provides in situations where dry pavement type traction is not available. It would therefore be desirable to provide a differential with substantially the same simplicity as the differentials just described, but which is quiet and smooth in its operation, so as to neither be heard by nor concern the average driver of a vehicle equipped with the locking differential.
Positive acting differentials are often installed as replacements for open differentials supplied as original equipment. It is desirable to provide a positive acting differential that can reuse a substantial portion of the original equipment differential, in particular, the original equipment differential case 54. Further, it is desirable to provide a positive acting differential that can be assembled into the original equipment differential case through an opening in the differential housing without removing the differential from the vehicle. This eliminates the significant expenses of providing a new differential case, of removing and installing the differential case, and of resetting the ring gear and drive shaft pinion gear backlash and alignment. As suggested by FIG. 2, differential cases 54 provide very little clearance for the assembly of parts within. The actual opening in a typical differential housing is more restrictive than that shown in FIG. 2. Parts for a positive acting differential must be designed both for proper operation and to facilitate assembly.
Referring again to FIG. 1, a typical assembly sequence concludes, after inserting all the pieces except for the pinion pin 34 and the second C clip 24, by shifting all the parts except for an unrestrained coupler 22 as far as possible to the side away from the unrestrained coupler. The objective is to create a space between the unrestrained coupler and the adjacent driver 30 for insertion of the final C clip. When the final C clip is installed, all parts can be moved to their final assembled positions. Insertion of the pinion pin 34 holds all parts in their assembled positions.
Recessed areas 86 are provided in the face of the drivers 30 in some prior art locking differentials. These recessed areas are to provide a space through which the C clips 24 may be inserted once the drivers and couplers have been assembled into the differential case and the axles put adjacent their final axial position. However the recessed areas 86 interrupt the continuity of the distribution of the teeth on the drivers, creating a lack of symmetry therein. The forces carried by the teeth on one side of the driver must be balanced by forces in the reduced number of teeth on the opposite side. Consequently, the load on the teeth of the drivers and couplers is not equally distributed among the teeth. This also creates a cocking force between the drivers and respective couplers which, particularly for less than full tooth engagement, results in the plane of the drivers and couplers not remaining accurately parallel. Further, for any two engaging teeth, the load on the area of engagement of the teeth is not equally distributed over the engaging area. The net result of the asymmetry is faster tooth wear than necessary, and reduced resistance to abuse because of load concentrations. Also, recessed areas 86 have to be located at an axial position where the C clip 24 may be slid onto the C clip groove in the axle. This may not be the final position of the axle, but it must be in the insertable range of the axle into the differential assembly. This predetermines limits for the axial position of the recess and, in turn, constrains the relative thicknesses of the drivers and couplers, preventing the optimization thereof to equalize the strength of the drivers and couplers.
An improved design for a positive acting differential is described in U.S. Pat. No. 5,901,618, issued May 11, 1999, assigned to Vehicular Technologies, Inc., and incorporated herein by reference. This design, shown in FIG. 3a, employs a ring structure 58 fitted to each coupler 50 to prevent premature engagement of the adjacent driver 52. The spacers 56 are modified to provide a notch 68 to fit the spacers to the pinion pin 34. This causes the spacers to rotate with the pinion pin. A paddle 64 is added to the spacer to transmit the pinion pin motion to the ring structure with a predetermined amount of backlash. The length of the spacer is increased to provide for the depth of the notch. This increased length makes assembly of a positive acting differential in an original equipment differential case 54 even more difficult.
Accordingly, it is desired to provide a positive acting differential that can be assembled in an original equipment differential case while the differential housing remains assembled to the vehicle. Further, it is desired to provide a design that can accommodate assembly with spacers of substantially increased length. Still further, it is desired to provide a positive acting differential that eliminates the recess in the face of the drivers and the resulting undesirable load distributions.